Lignocellulose-containing biomass is an effective starting material for a useful alcohol, such as ethanol, or an organic acid. Lignocellulose-containing biomass includes woody biomass and herbaceous biomass. Lignocellulose-containing biomass, such as woody biomass, is mainly composed of cellulose, hemicellulose, and lignin. In order to produce a liquid fuel such as ethanol from lignocellulose-containing biomass, cellulose or hemicellulose is hydrolyzed (saccharified) into a constitutive monosaccharide, and the monosaccharide is converted into ethanol via fermentation. Cellulose is composed of glucose, and hemicellulose is mainly composed of arabinose and xylose. When producing ethanol with the use of lignocellulose-containing biomass, accordingly, it is preferable that xylose be effectively used as a fermentation substrate, in addition to glucose.
When producing ethanol from lignocellulose-containing biomass, production costs can be reduced if the saccharification process can be simultaneously carried out with the fermentation process without separating these processes from each other. Such technique is referred to as “the simultaneous saccharification and fermentation process.” The simultaneous saccharification and fermentation process requires the use of thermotolerant microorganisms capable of fermentation in the reaction temperature range for a carbohydrase (i.e., about 40 degrees C. or higher) and capable of using the xylose (pentose) as a substrate, in addition to glucose.
Examples of known thermotolerant yeasts include yeasts of the genus Kluyveromyces, such as Kluyveromyces marxianus. While a yeast of the genus Kluyveromyces is capable of ethanol fermentation with the utilization of xylose, the yield thereof has not been sufficient. For example, the results of functional analyses of the alcohol dehydrogenase gene of Saccharomyces cerevisiae (including a plurality of isomers) are reported in Non-Patent Documents 1 and 2. Patent Document 1 discloses a recombinant yeast strain capable of isomerizing xylose into xylulose, in which alcohol dehydrogenase activity has been reduced. Based on such finding, however, the influence of the deficiency or destruction of the alcohol dehydrogenase gene on the capacity for ethanol fermentation cannot be determined. In addition, functions of the alcohol dehydrogenase gene of a taxonomically different species (i.e., a yeast of the genus Kluyveromyces such as Kluyveromyces marxianus) cannot be evaluated.